Machines such as, for example, passenger vehicles, trains, marine vessels, construction equipment, excavating machines, etc., often have a combustion engine to power various operations of the machine. In the past, the power output of the combustion engine was usually mechanically coupled to traction devices (e.g., wheels or tracks) to propel the machine. In response to ever more stringent emissions requirements and design goals, however, different propulsion arrangements have been developed. Combustion engines are now commonly used to drive an electric machine, such as an inductance machine, a permanent magnet machine, or switched reluctance (SR) machine, which operates in a generating mode to energize a DC bus that is used to power an electric motor at the traction devices. This arrangement is advantageous because it permits running the combustion engine at a constant speed (i.e., a “sweet spot”) that minimizes harmful emissions and/or maximizes fuel economy.
One challenge posed by this arrangement is to keep the DC bus energized such that sufficient power can be provided to the electric motor under a variety of load conditions. If a switched reluctance generator is used, the respective phases of the generator must be “fired” at appropriate rotational angles to energize the DC bus (e.g., charge a capacitor). Alternatively, if operating in a “motoring” mode, the respective phases of the motor must be similarly “fired” at the appropriate time. This is usually accomplished by responsively switching on and off transistors or other switching elements of an SR power converter connected to the DC bus. The power converter is switched in accordance with optimum “firing angles”, or “turn on” and “turn off” angles (θon and θoff), of the respective phases of the SRG. In other words, θon and θoff correspond to optimum angular positions along the rotational path of the SRG at which the transistors of the power converter are switched on and off to draw electrical current from the respective phase coils of the SRG to energize the DC bus.
One system that controls the conduction angle of a switched reluctance generator is described in U.S. Pat. No. 7,071,659 issued to Torrey et al. on Jul. 4, 2006 (the '659 patent). The system of the '659 patent includes an SRG coupled to power a load via a DC bus. The system includes a voltage controller that monitors the DC bus voltage via a feedback loop and makes changes to the conduction angle of the SRG to compensate for variations in the bus voltage caused by varying load conditions.
Although the system of the '659 patent may adjust the conduction angle of the SRG in response to changes in bus voltage, the response of the system may be inadequate. Particularly, the system response may be slow, because the voltage controller takes into account only the bus voltage as an indicator of the load applied to the system. Therefore, if the system is subject to a sudden increase or decrease in loading, the bus voltage may decrease or increase suddenly, and the load may be temporarily supplied with inadequate or excess power until the system can bring the bus voltage back to a desired level. This transient response may lead to less than optimum operation and/or malfunctioning of the load (e.g., a motor or another electronic device).
The present disclosure is directed to overcoming one or more of the problems set forth above.